#Proper Leadwork: Bay Sizing Essentials—Max Dimensions, Falls & Drips
The long life of a lead roof depends on correct lead bay sizing. Keep sheets modest, add the right falls and place drips at sensible intervals, and the roof works with thermal movement instead of fighting it. This guide gives you practical bay width/length limits by application, how to set falls and drip centres, what codes to choose, and how to tie everything back into fixings, underlay and overflow strategy.
Clips, Nails & Straps
Lead Underlay & Support
Wood Rolls & Timber Cores
The Lead Lads: Services & Prices
Lead Lads Standards Hub
GNR Roofing: Leadwork Authority
Regulatory Standards (BS 6915)
Part 6 of our “Proper Leadwork: Technical Series”. For context, see Part 1 (codes), Part 2 (movement) and Part 5 (rolls).
1) Why bay sizing dominates lead design
Lead expands and contracts every day. Wide sheets concentrate stress at drips and corners; long sheets creep and ripple. Correct lead bay sizing breaks the surface into pieces that can move, drain and breathe—dramatically reducing fatigue and deformation. Smaller bays also keep drips crisp and reduce ponding risk.
2) Indicative maximum dimensions by application
Use the limits below as a practical starting point. Then adjust for exposure, pitch, drainage complexity and access. In severe coastal/upland sites, reduce maximum widths and/or step up a code.
| Application | Typical Codes | Indicative Max Bay Width | Indicative Max Bay Length | Notes |
|---|---|---|---|---|
| Flat/low-slope bays | 5–7 | 500–750 mm | Up to ~1500–2000 mm between drips | Increase falls as widths approach limits; use regular drips |
| Dormer roofs | 5–6 | 450–600 mm | Up to ~1500 mm typical | Visible zones favour tighter centres for aesthetics |
| Parapet/box gutters | 5–7 | 600–800 mm | Panel lengths set by outlets/sumps (use joints) | Provide ≥1:80 fall; design overflow route |
| Valleys | 6–7 | By hydraulic capacity | — | Protect outlets; consider splash/scour |
Limits are not excuses to go “as big as allowed.” Smaller, well-drained bays nearly always last longer.
3) Falls, drips & centres: setting the water path
Design the water path first. Then dimension bays to match. On low slopes, use frequent drips to shorten bay lengths and encourage flow. On long runs, add sumps/outlets and a safe overflow route.
Recommended falls (guide)
- Flat/low-slope bays: ≥ 1:80 (steeper where possible)
- Parapet/box gutters: ≥ 1:80 with sumps at sensible intervals
- Back gutters: ≥ 1:80 and protected outlets
Avoid “dead-flat.” Even small positive falls reduce dwell time and staining dramatically.
Drip spacing (indicative)
- Every 1500–2000 mm on low-slope bays
- At outlet/sump transitions in parapet gutters
- At geometry changes or where traffic/snow loads concentrate
As lead bay sizing increases, tighten drip spacing to keep movement per panel modest.
4) Code selection vs bay width
Code choice and bay width are linked. Code 5 works well for modest widths; Code 6 adds stiffness for wider bays and gutters; Code 7 is reserved for exceptional spans or exposure. But remember: better drainage and more bays often outperform heavier codes alone.
| Typical Bay Width | Suggested Code | Notes |
|---|---|---|
| ≤ 500 mm | 5 | Default for many dormers and small bays |
| 500–650 mm | 5–6 | Choose 6 in exposure or where falls are marginal |
| 650–800 mm | 6–7 | Pair with generous falls and frequent drips |
5) Sheet layout: joints, laps & symmetry
A thoughtful layout makes installation faster and the roof tidier. Keep bays symmetrical, align roll centres with façades, and offset laps to avoid capillary traps. Laps must be generous—and completely free of fixings.
- Symmetry: Centre lines on key axes; mirror bays left and right where visible.
- Laps: Low-slope bays: head laps ~100–150 mm, side laps ~60–75 mm (increase with exposure).
- Offsets: Stagger joints to avoid double seams concentrating in one line.
- Welts: Welt long edges in exposure to stiffen and combat capillary action.
Do not force sealant into laps “for security”—it can block sliding and create capillary traps.
6) Parapet & box gutters: widths, sumps & overflow
Gutters concentrate flow and debris, so lead bay sizing must balance capacity and movement control. Use stiffer codes, generous falls, and joints that are accessible for maintenance.
Widths & lengths
- Width: 600–800 mm typical (reduce in severe exposure or where debris risk is high).
- Length: Break long runs at sumps/outlets; add expansion joints as needed.
Sumps, outlets & overflow
- Provide sumps at logical intervals, not just at ends.
- Protect outlets with formed guards; avoid scouring at edges.
- Design a safe overflow path that discharges externally during blockages.
Back gutters at chimneys follow similar rules but at smaller scales. See Part 4 (chimney packages).
7) Dormer bays & visible façades
Dormers are close to eye level. Tighter centres and neat roll lines improve both performance and appearance. Keep bays 450–600 mm with Code 5–6, align roll centres to window spacing, and soften all visible radii and end closures.
- Centres: 500–550 mm reads best visually; reduce in exposure.
- Drips: Place where shadow lines fall naturally; consistent spacing looks deliberate.
- Edges: Welt long edges on exposed façades; keep verges crisp and parallel.
8) Decks, underlay & friction management
A smooth deck and the right underlay reduce abrasion, telegraphing and noise. They also help sheets slide quietly with temperature swings, which is vital when lead bay sizing pushes towards the upper limits.
- Deck: Planed, flat and clean—no proud fixings, splinters or knots telegraphing through.
- Underlay: Purpose-made low-friction lead underlay across the whole area, continuous under drips and rolls.
- Build-up: Verify VCL/ventilation (warm vs cold roof) to manage condensation and surface temperatures.
9) Fixings & restraint that allow movement
Clips, straps and nails resist wind uplift and keep edges tidy—but they must not pin laps. Keep fixings out of lap zones, set sensible centres, and use copper or stainless to avoid corrosion and galvanic issues.
| Detail | Fixings | Spacing | Notes |
|---|---|---|---|
| Roll edges & drips | Clips (Cu/SS) | 300–450 mm | Even spacing; allow longitudinal slide |
| Parapet/box gutters | Straps + clips | 300–400 mm | Increase in severe exposure |
| Verges/cappings | Clips + straps | 300–400 mm | Watch local uplift at edges |
Fix the substrate, not the lap. Laps must remain free to slide.
10) An 8-step bay sizing workflow
- Define application (bay, gutter, dormer, back gutter, valley).
- Choose code for span/exposure (5–7 typical).
- Set falls (≥1:80 wherever possible) and overflow route.
- Pick bay widths (450–800 mm range by application/exposure).
- Place drips every 1500–2000 mm or at geometry breaks.
- Lay out joints/laps (head 100–150 mm; side 60–75 mm) and keep laps unpinned.
- Specify fixings (clips/straps centres; materials) and underlay.
- QA symmetry, spacings, photos and handover notes.
11) Worked examples
Example A: Low-slope roof bay (urban)
Goal: Durable surface with minimal rippling.
- Code: 6 (span near 650–700 mm).
- Falls: ≥1:80; add drips every ~1800 mm.
- Laps: Head 120 mm; side 70 mm; welt long edges if exposed.
- Fixings: Clips @ 300–400 mm; no fixings in lap zones.
- Underlay: Low-friction; deck free of proud fixings.
Example B: Parapet gutter (exposed, leaf fall risk)
Risk: Debris blockage and splash at outlets.
- Width: 650–750 mm with Code 6; break runs at sumps.
- Falls: ≥1:80; add protected outlets; defined overflow route.
- Laps: Head 120–150 mm; side 70–75 mm with welt.
- Fixings: Straps + clips @ 300–350 mm in exposed zones.
Example C: Heritage dormer (visible façade)
Goal: Symmetrical rhythm with long service life.
- Centres: 500–550 mm; align to window axes.
- Code: 5; step to 6 if spans approach 600 mm.
- Drips: Position to maintain visual balance; soft radii at ends.
12) Common defects from over-wide bays
- Fatigue cracks at drips: Bays too wide or falls too shallow. Fix: Reduce width; add drips; improve falls; consider higher code.
- Rippled sheets & creep: No drips on long runs; clips over-tight. Fix: Introduce drips; reset clips to allow slide.
- Ponding & staining: Dead-flat zones; outlets undersized. Fix: Regrade falls; add/resize outlets and overflow.
- Capillary ingress at seams: Unwelted long edges in exposure. Fix: Welt; increase side laps.
13) On-site checklist (printable)
- Application defined (bay/gutter/dormer) ✅
- Code 5–7 chosen for span/exposure ✅
- Falls set (≥1:80) and overflow route designed ✅
- Bay width within limits (450–800 mm) ✅
- Drips placed every 1500–2000 mm (or closer) ✅
- Laps sized (head 100–150 mm; side 60–75 mm), laps unpinned ✅
- Fixings centred (300–450 mm) and correct materials ✅
- Deck smooth; low-friction underlay installed ✅
- Photos and notes for handover ✅
14) Useful links & where to buy
